Directing Cluster Formation of Au Nanoparticles from Colloidal Solution

Abstract

Discrete clusters of closely spaced Au nanoparticles can be utilized in devices from photovoltaics to molecular sensors because of the formation of strong local electromagnetic field enhancements when illuminated near their plasmon resonance. In this study, scalable, chemical self-organization methods are shown to produce Au nanoparticle clusters with uniform nanometer interparticle spacing. The performance of two different methods, namely electrophoresis and diffusion, for driving the attachment of Au nanoparticles using a chemical cross-linker on chemically patterned domains of polystyreneblock-poly(methyl methacrylate) (PS-b-PMMA) thin films are evaluated. Significantly, electrophoresis is found to produce similar surface coverage as diffusion in 1/6th of the processing time with an ∼2-fold increase in the number of Au nanoparticles forming clusters. Furthermore, average interparticle spacing within Au nanoparticle clusters was found to decrease from 2−7 nm for diffusion deposition to approximately 1−2 nm for electrophoresis deposition, and the latter method exhibited better uniformity with most clusters appearing to have about 1 nm spacing between nanoparticles. The advantage of such fabrication capability is supported by calculations of local electric field enhancements using electromagnetic full-wave simulations from which we can estimate surface-enhanced Raman scattering (SERS) enhancements. In particular, full-wave results show that the maximum SERS enhancement, as estimated here as the fourth power of the local electric field, increases by a factor of 100 when the gap goes from 2 to 1 nm, reaching values as large as 10 10 , strengthening the usage of electrophoresis versus diffusion for the development of molecular sensors.

Comparison of electric field enhancements: Linear and triangular oligomers versus hexagonal arrays of plasmonic nanospheres

“…Electrophoresis deposition (EPD) allows for spacing between nanospheres as low as ≈1 nm [33]. The fabrication method is similar to that described in Sec.…”

Electric field enhancement with plasmonic colloidal nanoantennas excited by a silicon nitride waveguide

“…1. According to the SEM images of a fabricated sample reported in [24][25][26][27], an experimentally achievable gap between the nanospheres with diameters of 23 nm is 1-2 nm. Accordingly, in this paper we assume a gap spacing of 2 nm and gold nanosphere diameter of 23 nm and both parameters are kept constant for all the cases reported throughout the paper.…”

“…Since we have maximum FE for the dimer's orientation perpendicular to the z axis ( 90 Here we closely simulate an achievable experimental system. The chemical assembly procedure of [25] and [27] involves depositing gold nanospheres with thioctic acids ligands from colloidal solution on a diblock copolymer thin film composed of poly(methyl methacrylate) and polystyrene (PS-b-PMMA). Nanospheres chemically attach to PMMA domains that are recessed due to the chemical functionalization process [27].…”

“…However, the challenge of fabricating systems with high reproducibility in SERS response across the sensor surface is not frequently addressed. To overcome this challenge, in our previous work [25], [27] an innovative method for fabricating self-organized clusters of metal nanospheres with nanometer gap spacing on diblock copolymer thin films was successfully investigated. Specifically, monodisperse, colloidal gold nanospheres are attached on chemically functionalized polymer regions on a self-organized diblock copolymer template to engineer a high density of hotspots over large areas while using low cost fabrication methods.…”

Terahertz multispectral imaging for the analysis of gold nanoparticles’ size and the number of unit cells in comparison with other techniques

“…Advances in the controlled assembly of nanoscale building blocks, such as gold and silver nanoscale spheres, and quantum dots (QDs) have resulted in functional devices, such as nano-optoelectronic components, bio-photonics, nanosensors, and novel contrast probes for molecular imaging. 1,2 In the assembled structures, the photophysical properties of nanomaterials are a function of the number and the size of the nanomaterials and the distances among them. For instance, the fluorescence lifetime of a quantum dot cluster depends on the number of the QDs, due to the energy transfer between them.…”